Riot police used pepper spray and batons to stop protesters from storming the Admiralty compound and lay siege to chief executive Leung Chun-ying's office, the media reported on Monday.

The escalation was announced at 9 pm, when the Hong Kong Students Federation representative Nathan Law Kwun-chung urged protesters to head towards the government offices.

He said there would be a "long journey" through the night, and called on the demonstrators to "walk" with them. Minutes after, hundreds of protesters wearing an assortment of hard hats and protective masks thronged around the government headquarters and Tamar Park and began trying to breach police lines at various points.

Police used pepper spray and baton charges to repel them, leaving some bloodied and requiring treatment by makeshift medics. Key areas of violence were Lung Wo Road and the walkways from Harcourt Road to government headquarters.

Police had earlier warned of resolute action to prevent any such blockade, while students leaders told activists to avoid "giving the police a legitimate reason to take violent actions" as they stepped up the protests.

About 3,000 police officers were posted in Admiralty, with a further 4,000 in Mong Kok, where protests have continued despite the clearance of the main protest zone last week.

Describing the rally as posing an "extremely high risk", Senior Superintendent Kong Man-keung of the police's public relations branch warned the public against attending. He said student leaders had urged people to bring supplies to the site.

"I emphasise that if anyone obstructs the police in execution of their duty, charge the police line violently, or attempt to block the central government office, police will take resolute enforcement actions," Kong said.

Oscar Lai Man-Lok, spokesperson for student group Scholarism, said the crowd was the biggest in at least two weeks. He told protesters to stick to their non-violent principles and not to provoke or charge at police.